The present invention relates to a transmitter and a method for gain control in a transmitter. In particularly, but not exclusively, the present invention relates to a transmitter and a method for gain control in a transmitter such as in a spread spectrum multiple access system using, for example code division multiple access (CDMA). The transmitter and method may be used in a cellular telecommunications network.
FIG. 1 shows a known transmitter of a mobile station used in a cellular telecommunications network. The transmitter 1 comprises an antenna 2 which is used to receive and transmit signals. It should be appreciated that the transmit part only of the mobile station is shown in FIG. 1. The signal to be transmitted can be regarded in the illustrated transmitter 1 as being two signals, one of which is the sine component and the other of which is the cosine component. These compounds are alteratively referred to as the I and Q components. The I and Q components are initially at a baseband frequency. The I and Q signals are in digital form initially and are converted to analogue signals by respective digital to analogue convertors (DAC) 3a and 3b. The output of each of the digital to analogue converters 3a and 3b is connected to a respective lowpass filter 4a and 4b. The lowpass filters 4a and 4b filter out undesired components which are introduced by the digital to analogue converters 3a and 3b. 
The output of each of the digital to analogue converters 4a and 4b are input to an IQ modulator 5. The IQ modulator 5 includes two mixers 5a and 5b which mix each of the I and Q signals with a signal from a first local oscillator 7 to provide resulting bandpass signals at an intermediate frequency. It should be noted that the signal which is mixed with the Q component is 90xc2x0 out of phase with the signal which is mixed with the I component of the signal. This 90xc2x0 phase delay is introduced by delay element 5c. The resulting I and Q signals which are now at the intermediate frequency are then summed by a summer 5d of the modulator 5 to provide a single bandpass signal.
The output of the summer 5d is input to a first amplifier 9 which amplifies the output of the summer 5d. The output of the first amplifier 9 is input to a first bandpass filter 11 which filters out any undesired components of the signal which have been introduced by the first amplifier 9. The output of the first bandpass filter 11 is input to a first gain control block 13 which applies a gain to the signal output by the first bandpass filter 11. The first gain control block 13 receives a control signal 13a which determines the amount of gain to be applied by the first gain control block 13.
The output of the first gain control block 13 is input into a mixer 6 which also receives an input from a second local oscillator 8. The output from the second local oscillator 8 is mixed with the output from the first gain control block 13 to provide an output signal which is at the radio frequency, i.e. the frequency at which the signal is to be transmitted by the antenna 2.
The output of the mixer 6 is input to a second bandpass filter 15 which filters out any undesired components introduced by the mixer 6. The output of the second bandpass filter 15 is input to a second amplifier 17 which amplifies the signal. The output of the second amplifier 17 is input to a second gain control block 10. The second gain control block 10 receives a control signal 12 which determines the gain to be applied to the signal. In particular, the second gain control block 10 varies the amount of gain applied to the input signal in dependence on the control signal 12. The output of the second gain control block 10 is input to a high power amplifier 14 which amplifies the signal by a fixed amount. The output of the high power amplifier 14 is output to the antenna 2 via a duplex filter 42.
However, it is often useful to be able to measure the power of the signal which is transmitted. Accordingly, a directional coupler 16 or similar device is provided. The coupler 16 allows a small proportion of the signal to be transmitted to be removed. The power level of that small proportion of the signal is measured using a radio frequency to DC rectifier 18, consisting of a diode and passive component(s). By suitable scaling, a voltage value indicative of the power level of the signal which is to be transmitted can be obtained.
The duplex filter 42 has a transmit portion 42b which is tuned to the radio frequency. The transmit portion 42b removes undesired components introduced by the transmissive chain. The receive frequency is different from the transmit frequency. The duplex filter 42 also has a receive portion 42a which is tuned to the receive frequency.
The signal to be transmitted may be either a speech or data transmission and may be a combination of the two, depending on the use being made of the transmitter. For convenience, any references hereinafter to the type of signal being transmitted will be termed speech mode and data mode of the transmitter. In speech mode, the required power of the transmitted signal may be relatively low, because the gain of CDMA systems is relatively high for low bit rate services. However, in data mode the required power of the transmitted signal may be relatively high, because the gain lowers when the user data rate is increased.
The transmitter 1 shown in FIG. 1 is not particularly suitable for a system which requires high power control accuracy and high power control dynamic range. One example of such kind of system is CDMA. This is because in a CDMA system, the mobile station transmitter will often operate at a relatively low power level. If the arrangement shown in FIG. 1 is used, the entire transmission chain, particularly the power amplifier, will still consume power even if the power level required for the transmitted signal is relatively low. This means that the average power consumption is high and the life of the battery between chargings is reduced.
Typically, in WCDMA systems, the information bit rate of the transmitted signal may be in the approximate range of 12.2 kbps for speech signals 144 kbps (or even up to 384 kbps) for data transmission.. It is understood that about 10.7 dB (10 log 144+12.2) less transmission power is needed for transmission at 12.2 kbps and for 144 kbps. This 10.7 dB difference in the power requirement decreases the efficiency of power amplifier in speech mode since the power efficiency of a power amplifier decreases at lower output powers.
Reference is now made to the arrangement shown in FIG. 2. FIG. 2 shows a transmitter 19 which is used for TDMA mobile stations and which is disclosed in U.S. Pat. No. 5,152,004. In the arrangement shown in FIG. 2, the signal which is to be transmitted and which is at the radio frequency is input to a power divider 20. The power divider 20 divides a signal into two parts. One part of the signal is input to an amplifier 22 whilst the other part of the signal is input to an attenuator 24. When a high power transmitted signal is required, the signal is amplified by the power amplifier 22 and output to the antenna 2. However, when the transmitted signal is to have low power, the power amplifier 22 is not used and the signal only passes through the attenuator 24 to provide a lower power signal. The lower power signal is output by the attenuator 24 to the antenna 2. Whilst power consumption is reduced, the transmitter 19 shown in FIG. 2 has the disadvantage that the output power level will not always have a smooth transition when a change is made from the path using the power amplifier 20 and the path using the attenuator 24. This is because the arrangement of U.S. Pat. No. 5,152,004 does not have any circuitry which can provide accurate and hence smooth power control when changes between the power amplifier and the attenuator paths take place. This gives rise to glitches (inaccuracies) in the power control of the signal to be transmitted which is disadvantageous.
The arrangement of U.S. Pat. No. 5,152,004 uses real time (analogue) feedback for the power control. Real time feedback is possible for narrowband systems (as TDMA usually is). However, for wideband systems (such as CDMA) analogue feedback would lead to problems, for example in stability. Thus non-real time (digital) feedback is preferred for wideband system (CDMA).
U.S. Pat. No. 5,661,434 (Fujitsu) discloses a transceiver for use in a wireless local area network which has two amplifiers connected in series. Where a lower level of amplification is required, one of the two amplifiers can be bypassed. This transceiver suffers from the same disadvantages as U.S. Pat. No. 5,152,004.
The signal which is modulated prior to transmission is generally modulated using a digital modulation method. When a linear (digital) modulation method (such as band limited QPSK) is used, if the transmitter is not linear, spectrum spreading to adjacent channels can occur. This can be a problem for CDMA systems. This leads to a reduction in the quality of the transmissions and can also reduce the system capacity. If the transmitter is linear or substantially linear, the problem of spectrum spreading to adjacent channels can be reduced. The linearity of the transmitter is largely dependent on the operating characteristics of the power amplifier. Highly linear power amplifiers could be used to reduce the amount of spectrum spreading to adjacent channels. However, the power efficiency of linear amplifiers is poor. Less linear amplifiers are more efficient and in particular consume less power for the required amplification. It has therefore been proposed to use nonlinear amplifiers but with compensation for the non-linearity of the amplifier.
One method of compensation is digital predistortion. With this method, before a signal is input to a power amplifier, it is predistorted in a nonlinear manner. This predistortion is the inverse of the distortion which is applied by the amplifier. Accordingly, the predistorted signal is input to the amplifier which provides a linear output. However, whilst this method provides improved power consumption if the signal is to be transmitted with a relatively high power level, the power efficiency is lower when the signal is transmitted with a lower power level. This is because the predistortion part of the transmitter consumes the same amount of power regardless of the power level of the signal to be transmitted. Since CDMA mobile stations will tend to use lower power levels, there may be little power saving as compared to simply using a linear power amplifier.
It is an aim of certain embodiments of the present invention to provide a transmitter which makes more efficient use of power and which avoids glitches in the power level of the signal to be transmitted.
According to a first aspect of the present invention, there is provided a transmitter comprising: an input for receiving a signal; gain control means for applying a first gain to the received signal; first path means for providing a second, relatively high gain for said received signal; second path means for providing a third, relatively high low gain for said received signal; transmitter means for transmitting a signal; and control means operable in use, to cause a received signal to pass through the gain control means and said first path means when a relatively high gain is to be applied to the received signal and to cause a received signal to pass through the gain control means and said second path means when a relatively low gain is to be applied to said received signal, wherein when a change is made from using one of said first and second path means to using the others of said first and second path means, the power of the signal transmitted by the transmitter varies by less than or equal to a predetermined amount.
It is possible to ensure that the power of a signal transmitted by the transmitter means remains substantially the same or only varies by a small amount. The glitches which would occur with the prior art arrangements can be avoided. The power of the signal transmitted may be identical before and after making a transition between the first and second path means or there may be a difference in the power level before and after making such a transition. This difference may be relatively small. The output power of the transmitter means is preferably monotonic, particularly when the power level is generally increasing or generally decreasing.
Embodiments of the present invention are particularly applicable to transmitters which have a high dynamic power range and small power control step size. The power step size is preferably equal to the predetermined amount. For example, the step size may be 1 dB.
Measuring means may be provided to provide a value indicative of the value of the power level of the signal to be transmitted by the transmitting means. The measuring means may take any suitable form and may for example be provided by a combination of coupling means and power measuring means.
The measuring means may provide a reference voltage value when a signal passes through the second path means and gain of the gain control means has been set at a predetermined level, and the control means, when the measuring means provides the reference value when a signal passes through the first path means, causes a received signal to pass through the second path means. The predetermined gain level may be the maximum gain of the gain control means.
When the change is made so that a received signal passes through the second path means, the gain of the gain control means may be set to the predetermined gain level. This may ensure that the power level of the output signal remains the same or of a similar value when a transition is made from the first path means to the second path means.
Preferably, when the measuring means provides a predetermined value when a received signal passes through the second path means, the control means causes a received signal to pass through the first path means. This may occur in a tuning mode of operation. Preferably, when a signal passes through said first path means and the measuring means provides the predetermined value, the corresponding gain value defines a reference gain value. Preferably, the gain of the gain control means is set at the reference gain value when the control means causes a received signal to change to the first path means.
The predetermined value of the measuring means is preferably the same as the reference value of the measuring means.
Preferably, when the control means subsequently cause a change from the first path means to the second path means, the gain value of the gain control means which causes a received signal passing through the first path means to provide the predetermined value at the measuring means is stored as a new reference gain value. A new gain reference value may be stored each time there is a change from the first path means to the second path means.
Preferably, when the control means subsequently causes the change from the second path means to the first path means, the value of the measuring means caused by a received signal passing through the second path means when the gain of the gain control means is at the predetermined level is stored as a new reference value. Again, it is preferable that the reference value be updated each time there is a change from the second path means to the first path means.
Preferably, a temperature sensor is provided and the control means is arranged to compensate the reference gain value for variations in the temperature. Preferably, a temperature sensor is provided and the control means is arranged to compensate the reference value of the measuring means for variations in temperature.
The gain of the first and/or second path means are preferably constant. Thus, the variation in the output power level of the signal can be simply controlled by the gain control means. However, the gain of the first and/or second path means may be variable.
Preferably, the power level of the signal transmitted by the transmitter is increased or decreased by a predetermined amount when changing between the first and second path means. For example, in a typical CDMA system, this may be of the order of 1 dB.
The first path means may comprise amplifier means for amplifying a received signal. Predistortion means may be provided for predistorting a received signal prior to the signal passing through said amplifier means, whereby said predistortion means is arranged to substantially compensate for non-linearity of said amplifier means, the control means being arranged if the power level of the signal to be transmitted by the transmitter means is below a predetermined level, the signal does not pass through said predistortion means and if the power level of the signal to be transmitted by the transmitter means is above a predetermined level said received signal passes through said predistortion means and said amplifier means. When digital predistortion is not used, the second path may be used.
Thus, the predistortion means is only used when the power level of the transmitted signal is relatively high and the non-linearity of the amplifier means is most likely to cause problems. The predistortion means thus compensates for the nonlinear characteristics of the amplifier means. If the amplifier means are nonlinear, more efficient use of power can be achieved. However, when the power level falls below a predetermined level, the predistortion means are not used, thus saving the power required to operate the predistortion means.
Preferably, bias control means are provided for controlling the biassing applied to the amplifier means, whereby when the power level of the signal to be transmitted by the transmitting means is above the predetermined level, then the amplifier means is controlled by the bias control means to operate non-linearly. This gets the maximum power efficiency out of the transmitter even allowing for the extra power consumption required by the predistortion means.
If the power level of the signal to be transmitted by said transmitter is below the predetermined level, the amplifier means are controlled by the bias control means to operate substantially linearly. Thus, the signal may pass through the amplifier which as it is controlled to operate in a linear fashion, gives a linear output. However, in some embodiments of the present invention, the bias voltage applied to the amplifier may be only to avoid significant temperature changes in the amplifier means between the amplifier means being last used prior to the switching to the bypass path and being next used.
The first path may comprise a plurality of amplifiers arranged in series; and the second path bypasses at least one of said plurality of amplifiers.
In this arrangement, a plurality of amplifiers are connected in series at least some of which are bypassed. For example, if three amplifiers are provided, one or two amplifiers could be bypassed to provide the second path whilst all three amplifiers could be bypassed to provide a third path. This arrangement has the advantage that more power consumption savings and increased power control range can be achieved. The number of amplifier stages which a signal passes through can thus be controlled.
Preferably, the transmitter which may be a radio frequency transmitter described hereinbefore can be included in the mobile station. The mobile station may be arranged to work in a spread spectrum communications system. That spread spectrum communications system may use code division multiple access.
According to a second aspect of the present invention, there is provided a method for controlling gain of a transmitted signal comprising the steps of: receiving an input signal; applying a first gain to the input signal; causing a received signal to pass through a first path providing a second, relatively high, gain when a relatively high gain signal is required and causing a received signal to pass through second path means providing a third, relatively low, gain when a relatively low gain is required; and
controlling the gain applied to the input signal so that when a change is made using one of the first and second paths to using the other of said first and second paths, the power of the transmitted signal varies by less than or equal to a predetermined amount.
According to a third aspect of the present invention, there is provided a transmitter comprising an input for receiving a signal; first path means for providing a first, relatively high gain for said received signal; second path means for providing a second, relatively low gain for said received signals; and transmitter means for transmitting a signal, wherein said received signal comprises speech signals or data signals or a combination of speech and data signals.
Preferably the content of the received signal may include either a speech signal, a data signal, or a combination of speech and data signals.